chore: Prepare towards PHP8.4 compatibility

[sbuerk]

This pull-request contains three commits:

chore: fix latest php-cs-fixer findings

This ensures clean cgl state using the current php-cs-fixer version and ruleset.

chore: Mitigate PHP8.4 deprecation warnings (#570)

This change replaces implicitly nullable method arguments by making them explicitly nullable to mitigate PHP8.4 deprecation warnings.

chore: Enable php-cs-fixer rules to avoid implicitly nullable method arguments

This change enables two rules in the php-cs-fixer ruleset to guard against future implicitly method arguments and ensures to use the ? nullable operator for nullable methods and properties.

Resolves: #570